Research

I combine mathematical theory with field and laboratory experiments to investigate how species respond to environmental variability arising in the abiotic environment and via biotic interactions.

1. Ecological and evolutionary responses to environmental variation

Eco-evolutionary dynamics under variable thermal regimes

Species must cope with variable abiotic conditions, both seasonal and atypical; for example, due to climate change. To investigate how species' responses to environmental variation affect their ecological dynamics, I combine laboratory and field experiments with mechanistic mathematical models to predict insect population dynamics in seasonally-varying environments. Using case studies such as bordered plant bugs (Largus californicus) in coastal California (Johnson et al 2015), I quantify temperature responses of life history traits (reproduction, maturation, mortality) and interaction parameters (competition coefficients), parameterize mechanistic models, and validate model predictions (panels g,h) using time-series data (panels i,j) collected in the field (photo).

From an evolutionary perspective, I am developing models investigating how ectotherms such as insects evolve in response to variable thermal regimes and how eco-evolutionary feedbacks drive population dynamics. Our quantitative genetics model successfully captures the thermal response curves of tropical, Mediterranean, and temperate insects and predicts its evolution (Amarasekare & Johnson 2017). We are currently developing an adaptive dynamic framework (Johnson & Amarasekare in prep.) for investigating when evolution can 'rescue' species from extinction due to climate change (panel a) or facilitate invasion of novel habitats such as tropical invaders of temperate habitats (panel b).

Phenological responses to environmental variability

Species exhibit variable responses to abiotic conditions that drive changes in phenology - the timing of biological events - that can result in novel species interactions. With Jonathan Levine and collaborators, I am currently studying how phenology influences species' fitness, interactions, and demography via two large-scale field experiments in the Swiss Alps. (i) We quantify flowering phenology and reproduction in response to snowmelt manipulations (photo). (ii) Migration in response to climate change can lead to novel plant-pollinator interactions, which we study by transplanting whole plant communities along an elevation gradient. PhD student Sarah Richman and I are evaluating how phenology, pollinator visitation, and seed set of plants vary along the gradient in response to both native local competitors and novel pollinators (scenario 1) and novel competitors and pollinators (scenario 2).

2. Eco-evolutionary dynamics of species interactions

Consumer-resource oscillations and community assembly

Antagonistic interactions are prone to oscillations (top panel) that can result in stochastic extinction.Interaction strength, quantified by the attack rate, is a long-standing metric for predicting consumer-resource oscillations. We show that tension between positive feedback due to the multiplicative effect of the attack rate and the handling time and negative feedback due to self-limitation drives oscillations and derive a new metric for predicting oscillatory tendency (Johnson & Amarasekare 2015). Tests of the metric with data show that 23% of species exhibit high oscillatory tendency, consistent with time-series data. Recent interest in 'systemic selection' at the community level due to collapse of unstable modules has motivated us to study how coevolution of attack rates and handing times shapes community assembly (e.g., pairwise interactions to tri-trophic chains to foodwebs, bottom panels) in the face of population-level constraints imposed by stochastic extinction of highly-oscillatory consumer-resource interactions (Johnson and Amarasekare in prep.).

Antagonism, competition, and the evolutionary ecology of mutualism

Mutualisms are now recognized as consumer-resource interactions involving reciprocal exploitation that yield net benefits to each species. The exploitative nature of mutualism can drive antagonism as well as competition for partner commodities; e.g., nectar or pollen transfer in pollination mutualisms. I study how antagonism and competition shape the eco-evolutionary dynamics of mutualism. First, I investigate interplay between mutualism and antagonism arising via mutualist ontogeny. Many pollinators (e.g., Lepidopterans) have herbivorous larvae, sometimes of the same plant. Using Manduca sexta and its host plants Datura wrightii and D. discolor as a model system, PhD student Gordon Smith and I conducted laboratory experiments (Smith et al., 2017) which I used to parameterize models investigating the ecology and evolution (Johnson et al., in prep.) of mutualism and antagonism by partners themselves.

Second, I derive frameworks elucidating how competition for shared resource (panel a) or service (panel b) commodities promotes or erodes coexistence of mutualists sharing partners; e.g., plants competing for pollinators (Johnson & Amarasekare 2013; Johnson in review). Models predict that mutualists that are most limited by the same commodities cannot coexist, coexistence requires stronger intra- to inter-specific negative feedback. I am currently applying Coexistence Theory to mutualism such that it is the interplay of competition and mutualism that modulates both the niche and fitness differences between species that ultimately determine whether species coexist (Johnson in prep.).